While substantial effort and attention continues toward the development of newer and more sustainable energy supplies, the conservation of energy by increased energy efficiency remains crucial to the world's energy future. According to an October 2010 report from the U.S. Department of Energy, heating and cooling account for 56% of the energy use in a typical U.S. home, making it the largest energy expense for most homes. Along with improvements in the physical plant associated with home heating and cooling (e.g., improved insulation, higher efficiency furnaces), substantial increases in energy efficiency can be achieved by better control and regulation of home heating and cooling equipment. By activating heating, ventilation, and air conditioning (HVAC) equipment for judiciously selected time intervals and carefully chosen operating levels, substantial energy can be saved while at the same time keeping the living space suitably comfortable for its occupants.
Programmable thermostats have become more prevalent in recent years in view of Energy Star (US) and TCO (Europe) standards, and which have progressed considerably in the number of different settings for an HVAC system that can be individually manipulated. Some programmable thermostats have standard default programs built in. Additionally, users are able to adjust the manufacturer defaults to optimize their own energy usage. Ideally, a schedule is used that accurately reflects the usual behavior of the occupants in terms of sleeping, waking and periods of non-occupancy. Due to difficulty in programming many thermostats, however, may schedules do not accurately reflect the usual behavior of the occupants. For example, the schedule may not account for some usual periods of non-occupancy. Additionally, even when a suitable schedule is programmed into the thermostat, inevitably there are departures from usual behavior. The user can manually set back the thermostat when leaving the house and then resume the schedule upon returning, but many users never or very seldom perform these tasks. Thus an opportunity for energy and cost savings exist if a thermostat can automatically set back the setpoint temperature during time of non-occupancy.
U.S. Patent Application Publication No. 2010/0019051 A1 discusses overriding of nonoccupancy status in a thermostat device based upon analysis or recent patterns of occupancy. The publication discusses a “safety time,” for example during the nighttime hours in a hotel or motel room, during which requirements to maintain a condition of occupancy are relaxed based on pattern recognition analysis. A “hysteresis” period of typically less than a few minutes can be built into the motion sensor to establish occupancy for some period after any motion is detected or signaled. An increased hysteresis period can be used during safety times such as during the evening and night hours. The focus is mainly on reliably detecting when occupants return from an absence.